Field of the Invention
Exemplary embodiments of the present invention relate to a communication system and, more particularly, to a codec apparatus and method for coding/decoding a speech and audio signal in a communication system.
Description of Related Art
In a communication system, active research are being carried out in order to provide users with various types of Quality of Services (hereinafter referred to as ‘QoSs’) having a high transfer rate. In this communication system, schemes for transmitting data having various types of QoSs through limited resources rapidly are being proposed. With the recent development of networks and the recent increase of user demands for high quality service, speech/audio codecs have been developed as schemes for compressing and transmitting a speech and audio signal in a network.
Meanwhile, in order to transmit and receive a speech and audio signal over a digital communication network, an encoder for compressing the speech and audio signal converted into a digital signal and a decoder for restoring the speech and audio signal from the compressed signal are essential to a communication system. In general, the encoder and the decoder are collectively called a codec or coder. As an example of a proposed codec, one of the most widely used speech/audio codec techniques is a Code Excited Linear Prediction (hereinafter referred to as ‘CELP’) codec. The CELP codec is represented by a synthesis filter indicative of the constellation of a speech and audio signal and an excited signal corresponding to the input of the synthesis filter.
Furthermore, the CELP codec includes an Adaptive Multi-Rate (AMR) codec, that is, a narrowband codec, and an Adaptive Multi-Rate WideBand (AMR-WB) codec, that is, a wideband codec. In an encoder, each of the barrowband AMR codec and the wideband AMR-WB codec extracts the coefficient of the synthesis filter from an input signal of one frame corresponding to 20 msec, splits the one frame into subframes of 5 msec, calculates a pitch index and the gain of an adaptive codebook and a pulse index and the gain of a fixed codebook, quantizes the calculated parameters, and sends the quantized parameters to a decoder. In the decoder, each of the barrowband AMR codec and the wideband AMR-WB codec generates excited signals by using the pitch index and the gain of the adaptive codebook and the pulse index and the gain of the fixed codebook and restore a speech and audio signal by filtering the excited signals through the synthesis filter.
The wideband AMR-WB codec further sends information on a Voice Activity Detection (VAD) flag and a Long Term Predictor (LTP) filter flag as a transmission parameter. The VAD flag indicates whether a VAD function operates or not, and the LTP filter flag indicates whether a Low-Pass Filter (hereinafter referred to as an ‘LPF’) will be applied to an adaptive codebook excited signal or not. The LTP filter flag is transmitted in modes other than two lower modes having a low bit rate, from among the 9 bit rate modes of the wideband AMR-WB codec.
Meanwhile, the barrowband AMR codec, that is, a narrowband codec, codes a signal of a 300˜3400 Hz band, whereas the wideband AMR-WB codec, that is, a wideband codec, codes a signal of a 50˜7,000 Hz band. That is, the wideband codec processes a signal having a frequency band twice wider than that of the narrowband codec. Thus, in the case of a wideband signal, a harmonic component on the spectra of a signal represented by an adaptive codebook parameter may appear in all frequency bands of 50˜7,000 Hz. However, the wideband signal includes a harmonic component that appears only in a relatively low frequency band, but also includes a harmonic component that is weak or does not appear in a high frequency band. In order to represent a signal having a weak harmonic component in a high frequency band, the wideband AMR-WB codec extracts an adaptive codebook parameter by using the LPF. That is, the narrowband codec and the wideband codec, particularly, the wideband codec uses an adaptive codebook excited signal without change if a harmonic component on the spectrum of a speech and audio signal appears in all frequency bands, but uses an adaptive codebook excited signal filtered by the LPF if a harmonic component is weak in a high frequency band.
If a harmonic component is weak in a high frequency band as described above, however, the narrowband codec and the wideband codec, particularly, a wideband codec has to use an adaptive codebook excited signal filtered by the LPF and send information indicating whether the LPF has been applied or not, that is, information on the LTP filter flag, to the decoder. In this case, there is a problem in that 1 bit is necessary for each subframe, that is, 4 bits per frame, in order to send information on the LTP filter flag.
Accordingly, in order to provide a speech and audio service having high quality in a communication system, there is a need for a codec for coding/decoding a speech and audio signal with no need for a narrowband codec and a wideband codec, particularly, the wideband codec to send additional information, for example, information on the LTP filter flag.